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The main objective of the study was to determine the kinetics of the polyphenol oxidation reaction in cocoa beans during air drying at various air temperatures and humidities. The temperatures used were between 40 and 60°C and the relative humidities were between 50 and 80%. The higher the temperature and relative humidity of the drying air, the lower the amount of polyphenol residue in the cocoa beans, because of enzymatic oxidation of polyphenols. At higher drying temperatures, non-enzymatic oxidation of polyphenols could also occur. Computer simulation results gave rate constants for the polyphenol oxidation reaction (k 1 ) and the condensation reaction (k 2 ), at various air conditions, in the range of 0.055-0.200 and 0.136-0.155 h )1 respectively. The activation energies obtained for the polyphenol oxidation reaction were in the range of 27 800-30 312 J K )1 mol )1 . The reaction kinetics of the enzymatic browning reaction fitted a pseudo first-order reaction.

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Computational fluid dynamics (CFD) modeling of spray dryers requires a simple but sufficiently realistic drying model. This work evaluates two such models that are currently in discussion; reaction engineering approach (REA) and characteristic drying curve (CDC). Two versions of the CDC, linear and convex, drop in drying rate were included. Simulation results were compared to the overall outlet conditions obtained from our pilot-scale experiments. The REA and CDC with a linear drop in drying rate predicted the outlet conditions reasonably well. This is contrary to the kinetics determined previously. Analysis shows that the models exhibit different responses to changes in the initial feed moisture content. Utilizing different models did not result in significantly different particle trajectories. This is due to the low relaxation time of the particles. Despite the slight differences in the drying curves, both models predicted similar particle rigidity depositing the wall. For the first time in a CFD simulation, the REA model was extended to calculate the particle surface moisture, which showed promising results for wet particles. Room for improvement was identified when applying this concept for relatively dry particles.